Radioactivity and Particles
4 marks · NormalA sample containing a radioactive isotope has an initial activity of 200 Bq. The half-life of the isotope is found to be 15 minutes.
Calculate the activity of the sample after 30 minutes.
State the unit for activity.
Explain what is meant by the 'half-life' of a radioactive isotope.
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(a) 50 Bq
(b) Becquerel (Bq)
(c) The half-life is the average time taken for half the nuclei in a radioactive sample to decay.
Common mistakes: Not using exponential decrease; Incorrect use of units
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Marking points
M1 (a) · 1 mark · Recognises two half-lives elapsed (30/15 = 2). (follow-through)
A1 (a) · 1 mark · Calculates correct final activity (200 → 100 → 50 Bq). (follow-through)
B1 (b) · 1 mark · States unit is becquerel (Bq).
B2 (c) · 1 mark · Gives correct definition for half-life.
Worked solution
a) Each half-life is 15 min. After 15 min, activity = 200/2 = 100 Bq; after 30 min (two half-lives): 100/2 = 50 Bq.
b) The unit for activity is the becquerel (Bq).
c) The half-life of a radioactive isotope is the average time it takes for half of the radioactive nuclei in a sample to decay.